SOLDERING
by Harry Lythall - SM0VPO

Since starting this homebrew construction hamepage I have received several E-mail messages which show me that some of my readers are relative newcommers to radio construction, or are involved with educating newcommers. Some tips and basic information about tools, soldering and component identification will help many would-be constructors. If you are involved with education then please feel free to download/print this page as an aid. If you feel that I have missed anything out then please E-mail me - constructive suggestions are always welcome.

How to solder

When assembling PCBs it is essential that all soldered joints are sound and error free. If you are inexperienced at soldering then I suggest you take a piece of vero-board or an old printed circuit board and make a few practice joints until you are confident that you can solder without errors. Stripping the components from an old transistor radio is excellent practice. The following information may help you.

To make a good soldered joint the component and/or PCB leads must be free from dirt and grease. A pencil eraser is ideal for this purpose. Using a sharp knife, make a small cut in the eraser, then pass the component lead through the slit whilst applying pressure to the sides of eraser. Simply rubbing PCB solder pads with the eraser is sufficient clean them.

Insert the component in the PCB and bend the ends of the leads over so that they lie flat on the PCB and in the direction of the PCB track leading from the pad. Cut away surplus component wire with sharp wire-cutters. It is most important to make a good mechanical joint before soldering.

If two components are to fitted to the same PCB pad then bend the component leads in opposite directions so that they lie side-by-side.

When making a soldered joint the object is to get the solder to flow over the component lead and the copper track of the PCB. Excessive solder looks unsightly and can hide bad joints. Excessive solder may also form a bridge between adjacent copper pads on the PCB resulting in "short-circuits". The correct amount of solder is used when you can see an impression of the component lead in the solder, yet the solder has flowed over the complete component lead and the PCB pad. The joint should have a smooth mirror-like surface without spikes or mounds.

Before applying the solder to the joint, wipe the tip on a slightly moist sponge to remove oxides and excess solder. Apply a very small amount of solder to the tip of the iron so that there is a very thin film of solder on the bit. Apply the soldering iron to the joint so that it touches both the component lead and the PCB solder pad. Apply the solder wire to the component lead and NOT to the iron. When the joint has heated sufficiently the solder will flow over the joint. Remove the solder and iron simultaneously and leave the joint to cool naturally. Do not blow the joint to cool it or try to move it until the solder has become solid or a poor joint will result.

The soldering iron should not be too hot. The soldering iron is too hot if your soldered joints appear to have black charred deposits around them or, in extreme cases, PCB pads are lifted from the board. You should use 60/40 solder which is an alloy of Tin and Lead with a flux core in the centre. This is the optimum solder type which has a very narrow temperature window for the "plastic" state. Other solder alloy combinations are available, but these will require a little more care.

The graph above shows why the temperature of the soldering iron is most important, and why we use 60/40 solder. I usually run my soldering iron at about 220 - 280C which is ideal for delicate printed circuit boards. SRPB is easily damaged, so under 250C is a requirement for delicate tracks. Higher temperatures will destroy the glue under the copper, so wide tracks should be soldered as quickly as possible if the iron must be run over 250C. Many fibre-glass will tolerate up to about 360C, but this sort of temperature should be regarded as a killer for all PCBs as it is ridiculously high. You should never exceed 300C, except when you are soldering mechanical items, such as copper heat sinks. By the way, if you have one of these new soldering iron thermometers, then don't take the temperature reading for granted!

How to desolder

De-soldering components frequently damages PCBs, so think TWICE and solder ONCE. A PCB is quite expensive to make and its value is also equal to all the other components that are mounted on it, plus your working time. If you have to remove a component then it is best to sacrifice the component and save the board. Cut all the component leads off so that the component falls off the PCB. Then remove the component leads one-by-one. This is especially true for ICs. Even a microprocessor chip may only have a value of $5 but may be mounted on a PCB worth well over $100.

Rules for soldering

Here are a few simple rules for soldering:

Have fun, de HARRY, Lunda, Sweden.

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